This research applies the physiologicval theory of nerve-cell functioning to investigate the behavior of neuronal circuits in the hypothalamus of mammals that have been posutlated to control thermoregulation. specifically, it furnishes quantitative tests of the hypothesis that the distribution of thermosensory fibers is the chief determiner of the first afferent stage of hypothalamic thermoregulatory circuits, and that the subsequent hypothalamic circuirty is conserved among mammalian groups having different environmental thermal stresses. The computer studies make specific predictions of neuronal firing rates as well as metabolic rate and other heat-loss and heat-gain motor responses as a function of skin temperature and experimental obsevations. These investigations furnish a physiologically rigorous basis for understanding central neural control of thermoregulation and its relationship to environmental factors and developmental mechanisms.